Ductility and dynamic strain aging in rapidly solidified aluminum alloys
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I.
INTRODUCTION
RAPID solidification techniques have
led to the development of a new class of high-temperature aluminum alloys having useful mechanical properties up to 600 K. However, some specific features of these properties have not yet been completely clarified. In particular, the ductility of rapidly solidified aluminum alloys is significantly lowered in a temperature range which is of interest for practical applications. The present work describes this problem, and an attempt is made to interpret this ductility trough, which appears to be a general phenomenon. For this purpose, the microstructure and mechanical properties of two commercial alloys, A1-Cr-Zr and A1-Fe-V-Si, have been investigated. The results and analysis reported here strongly support the hypothesis of Skinner et al.,U] where the loss of ductility stems from dynamic strain aging (DSA), In fact, anomalies in the mechanical properties associated with DSA and jerky flow (the Portevin-Le Ch~telier effect) around room temperature have been extensively described in the literature for conventional aluminum alloys. In rapidly solidified alloys, slow diffusing elements are introduced to avoid a rapid coarsening of the dispersoids. As a consequence, phenomena related to DSA occur at higher temperatures and are enhanced by the extension of solid-state solubility during rapid solidification. The microstructures and the tensile behavior of the two alloys have been examined. Strain-rate change tests have been performed in order to check whether a reduction in the strain-rate sensitivity (SRS) occurs in the temperature range of the ductility dip. Static aging experiments were also undertaken in the same temperature range to confirm the hypothesis that, in both the investigated alloys, a solute element can diffuse toward "arrested" dislocations. Finally, these results are discussed and an attempt is made, through semiquantitative arguments, to identify the solute species responsible for DSA in each alloy.
E. BOUCHAUD and H. OCTOR, Engineers, are with the Office National d'Etudes et de Recherches A6rospatiales (ONERA). L. KUBIN, Research Director, is with the Centre National de la Recherche Scientifique, Unit6 Mixte CNRS-ONERA-BP72, 92322 Chtitillon Cedex, France. Manuscript submitted March 28, 1990. METALLURGICAL TRANSACTIONS A
II. MATERIALS AND EXPERIMENTAL TECHNIQUES The two materials investigated in the present study are the A1-Cr-Zr Alcan, Banbury, Oxon, U.K., alloy121 and the A1-Fe-V-Si (or FVS0812) alloyt3'4] produced by AlliedSignal, Morristown, NJ. Both alloys were obtained from melt-spun ribbons which were comminuted and consolidated by extrusion. The dimensions, as well as the average chemical composition of the two initial products, are indicated in Table I. Table II shows the chemical compositions of the matrices in the "as-received" extruded conditions as determined by atom probe microanalysis. The microstructures were investigated by optical microscopy, X-ray diffraction, X-ray microprobe analysis, transmission electron microscopy (TEM), a
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